1. Field of the Invention
The present invention relates to a micromembrane pump and, in particular, to a micromembrane pump comprising a pump membrane, a pump body and inlet and outlet openings provided with passive non-return valves.
2. Description of Prior Art
According to the prior art a large number of different micromembrane pumps exists, the drive concepts used being predominantly electromagnetic, thermal and piezoelectric driving principles. Electromagnetic driving principles are described e.g. in E. Quandt, K. Seemann, Magnetostrictive Thin Film Microflow Devices, Micro System Technologies 96, pp. 451-456, VDE-Verlag GmbH, 1996. Thermal drive concepts are explained e.g. in B. Bustgens et al, Micromembrane Pump Manufactured by Molding, Proc. Actuator 94; Bremen 1994, pp. 86-90. EP-A-0134614 and H. T. G. Van Lintel et al, A Piezoelectric Micropump Based on Micrmachining of Silicon, Sensors & Actuators, 15, 1988, pp. 153-167, explain piezoelectric driving principles.
Piezoelectric drives are based on the use of piezoceramics causing a movement of the pump membrane and producing therefore a pumping effect in combination with a valve unit and a connection unit, respectively. There are several variations of piezoelectrically driven micromembrane pumps operating with active valves, with passive non-return valves or also with valveless fluidic connections. Such valveless fluidic connections are disclosed e.g. in A. Olsson et al: The First Valve-less Diffuser Gas Pump, Proceedings MEMS 97, pp. 108-113, Nagoya, Japan, 1997.
EP-A-0134614 describes a peristaltic pump making use of three piezomembranes, one piezomembrane being positioned at the inlet, the other one at the outlet and a further one between these two. On the basis of the periodic movement of the piezomembrane at the inlet and the movements of the piezomembranes at the outlet and in the middle, which are displaced in phase relative to the first-mentioned periodic movement, a pumping movement of the medium to be pumped is accomplished in the final analysis.
In addition, piezoelectric bending transducers, which are fixedly held on one side thereof, are known, the pump membrane being secured to a free end of these bending transducers. Such drive units are combined with a valve unit consisting of passive non-return valves.
Like the above-mentioned peristaltic pumps, the afore-mentioned valveless piezomembrane pumps make use of a piezomembrane as a drive unit, a fluidic connection unit being used, which consists of conically tapering channels with different flow resistances. By means of these pyramidal diffusers a direction-dependent flow resistance is defined, which produces a pumping effect in one direction. Like the other micropumps, also such a micropump including a valveless connection unit can build up a counterpressure during operation; this counterpressure can, however, no longer be maintained when the drive unit is switched off.
A known micromembrane pump which has an electrostatic drive is described in R. Zengerle: Mikromembranpumpen als Komponenten fur Mikro-Fluidsysteme; Verlag Shaker; Aachen 1994; ISBN 3-8265-0216-7, and shown in DE 41 43 343 A1. Such a micropump is shown in FIG. 1.
The micropump shown in FIG. 1 consists of four silicon chips, two of these chips defining the electrostatic actor consisting of a flexible pump membrane 10 and a counter-electrode 12 which is provided with an insulating layer 14. The two other silicon chips 16 and 18 define a pump body having flap valves 20 and 22 arranged therein. A pump chamber 24 is formed between the pump body, which is defined by the silicon chips 16 and 18, and the flexible pump membrane 10, which is connected to the pump body along the circumference thereof. A spacer layer 28 is arranged between the suspension devices 26 of the flexible pump membrane 10 and the counterelectrode.
When an electric voltage is applied to the electrostatic actor, the elastic pump membrane 10 is electrostatically attracted to the rigid counterelectrode 12, whereby a negative pressure is generated in the pump chamber 24, this negative pressure having the effect that the pump medium flows in via the inlet flap valve 22, cf. arrow 30. When the voltage has been switched off and the charge has been balanced by short-circuiting the electrodes, the pump membrane will relax and displace the pump medium from the pump chamber via the outlet flap valve 20.
In contrast to the above-described electrostatic drives, the pump membrane of a piezoelectrically operated micropump is moved by piezoelectric forces, a piezoelectric crystal being connected to the pump membrane. The application of an electric voltage to the piezoelectric crystal causes a contraction or an elongation of the crystal and therefore a bending deformation of the membrane, which, together with a valve unit of the type shown e.g. in FIG. 1, finally produces a pumping effect. With the exception of the different drive means, also a piezoelectrically driven micropump could have the structural design described in FIG. 1.
The above-described electrostatically driven micromembrane pumps have a plurality of disadvantages when used in the form shown e.g. in FIG. 1.
Due to the small stroke of the electrostatic actor, typically 5 .mu.m, and the comparatively large pump chamber volume, the height of the pump chamber being typically 450 .mu.m, such a known pump has a very small compression ratio. The term compression ratio stands for the ratio of the displaced pumping volume to the total pump chamber volume. Due to this small compression ratio, it is impossible to convey compressible media, such as gases, since the compressibility of such media normally exceeds the compression ratio of the pump.
Furthermore, the pump chamber of the known pumps described has a geometry which is disadvantageous as regards fluid dynamics and which is, moreover, not bubble tolerant. Inclusions of air in a fluid pump medium accumulate in the pump chamber and, due to their comparatively high compressibility, they cause a substantial deterioration of the pumping characteristics. In addition, a self-priming behaviour cannot be achieved due to the poor compression behaviour. Due to the production process used, the pump membrane of the known micropumps is, in addition, in electrical contact with the medium conveyed. Since voltages in the order of 200 V occur at the actor during operation, substantial electric potentials may exist in the pump medium in the case of failure, and, depending on the respective case of use, these electric potentials may cause a malfunction of external components. In addition, known micro-pumps are mounted by glueing individual chips according to the prior art known at present, this kind of mounting being incapable of satisfying the requirements which have to be fulfilled for an efficient production.
Also existing piezoelectric micromembrane pumps show most of the above-mentioned disadvantages. Fundamentally, a substantial advantage of the piezoelectric micropump in comparison with the electrostatic micropump is to be seen in the possibility of driving the actor also by voltages which are lower than 200 V. Hence, the pumping rate can be adjusted via the frequency as well as via the driving voltage, a circumstance which may result in substantial simplifications as far as the driving electronics is concerned.
DE 195 46 570 C1 shows a micromembrane pump defined by a structured silicon plate and a glass sheet connected thereto, non-return valves being formed in the structured silicon plate. DE 694 01 250 T2 shows a fluid pump without passive non-return valves. The pump disclosed in this publication has a small pump chamber so that the pump can be filled due to capillary forces alone.
It is the object of the present invention to provide a micromembrane pump which eliminates the above-mentioned disadvantages of the prior art, which permits compressible media to be conveyed, and which shows a self-priming behaviour and is bubble-tolerant. It is a further object of the present invention to provide a method of producing a pump body for such a micromembrane pump.
In accordance with a first aspect of the present invention, this object is achieved by a micromembrane pump comprising a pump membrane which is adapted to be moved to a first and a second position with the aid of a drive means, and a pump body connected to the pump membrane so as to define a pump chamber between these two components, the pump body being defined by two semiconductor plates having each formed therein a valve seat and a valve flap which are formed integrally with the respective semiconductor plate, the two semiconductor plates being connected in such a way that a respective passive non-return valve is defined by a valve seat of one semiconductor plate and by a valve flap of the other semiconductor plate, one of these passive non-return valves being arranged in an inlet opening penetrating both semiconductor plates, whereas the other of these non-return valves is arranged in an outlet opening penetrating both semiconductor plates. The pump membrane increases the volume of the pump chamber by a stroke volume when moving from the first to the second position and reduces the volume of the pump chamber by this stroke volume when moving from the second to the first position. According to the present invention, the ratio e of the stroke volume to the volume of the pump chamber satisfies the following equation, when the pump membrane is at the first position: ##EQU2##
wherein p.sub.0 is the atmospheric pressure, .GAMMA. the adiabatic coefficient, and .DELTA..sub.Pcrit the maximum pressure value which depends on the valve geometry and on the wetting of the valves and which is required for opening the valves.
The present invention is based on the finding that the compression ratio, i.e. the ratio between the displaced pumping volume and the total pump chamber volume, is a decisive criterion of the behaviour of a micromembrane.
Such a compression ratio can be achieved in the case of one embodiment by using an electrostatic drive means as a drive means whose electrostatic actor is defined by the pump membrane and a counterelectrode, the pump membrane having a substantially planar structural design of such a nature that it abuts on the pump body at the first end position outside the inlet opening and the outlet opening.
Such a compression ratio can also be achieved in an advantageous manner by a pump body defined by two semiconductor plates which are connected at the main surfaces thereof, the inlet and the outlet valve being each defined by valve seats and valve flaps etched in these semiconductor plates and arranged in valve wells defining the inlet opening and the outlet opening. According to preferred embodiments, the semiconductor plate facing the pump membrane is thinned so as to define a shallow valve well between the semiconductor-plate surface facing the pump membrane and the valve flaps.
In accordance with a second aspect of the present invention this object is achieved by a method of producing a pump body of the type in question, the first step of this method being the step of structuring a respective first main surface of a first and of a second semiconductor plate for defining a valve flap structure of the inlet valve and a valve seat structure of the outlet valve in the first disc and a valve flap structure of the outlet valve and a valve seat structure of the inlet valve in the second disc. Following this, a valve flap well structure and a valve opening well structure are formed in a predetermined relationship with the valve flap structures and the valve seat structures in a respective second main surface of the first and of the second semiconductor plate. The first main surfaces of these first and second semiconductor plates are then connected in such a way that a respective valve flap structure is arranged in a predetermined relationship with a respective valve seat structure. Following this, at least one of the semiconductor plates is thinned starting from the second main surface, whereupon the respective second main surfaces of the first and of the second semiconductor plate are etched at least in the area of the valve flap well structure and of the valve opening well structure so as to expose the valve flaps and open the valve seat. The two last-mentioned steps of thinning and etching can be carried out in one operation.
The present invention provides a micromembrane pump showing a self-priming behaviour, this micromembrane pump being suitable for conveying compressible media and being, in addition, bubble-tolerant. Hence, the present invention provides a micropump offering manifold new possibilities of use because it is easy to handle. Furthermore, due to the stacklike structural design, the pumping concept according to the present invention is generally suitable for final assembly at wafer level; in comparison with numerous other concepts, it is therefore very adavantageous from the point of view of production engineering.
Further developments of the present application are disclosed in the dependent claims.